Ceramic shell mold facecoat and core coating systems for investment casting of reactive metals

ABSTRACT

An yttria-based slurry comprising a dense grain yttria powder and a non-aqueous-based binder is used as a mold facecoat and corecoat for investment casting of reactive metals such as titanium and titanium alloys.

BACKGROUND OF THE INVENTION

The present invention relates to mold facecoats and corecoats for use inthe fabrication of molds for casting reactive metals, particularlycomplex shapes thereof.

Melting and investment casting of reactive metals, such as titanium ortitanium alloy, is made difficult due to the reactive metal's affinityfor elements such as oxygen, nitrogen and carbon. At elevatedtemperatures, the reactive metals tend to react with almost any type ofcontainment incorporating such elements. For example, at elevatedtemperatures during investment casting, solidification and cooling, aTi-6Al-4V alloy reacts with oxygen and/or most oxide ceramics to form anoxygen enriched surface layer. This surface layer, commonly referred toas an "alpha-case" or a "coarse basket weave," can be brittle and istherefore detrimental to the mechanical properties of the casting andthus must be removed.

Typically, removal of oxygen or interstitial element enriched surfacematerial is accomplished by mechanical or chemical means such aschemical milling in an acid bath. This process, however, is notstraightforward, since the thickness of the alpha-case on an as-castcomponent varies for each section of the component depending on thethickness and solidification rate of the section. On the other hand,chemical milling removes surface material at an essentially uniform rateregardless of the section's thickness. Consequently, numerous iterationsmay be necessary to determine the proper wax pattern die size which mustbe utilized to generate a chemically milled component having therequired finished product dimensions.

In investment casting, mold/metal reactivity traditionally has beenreduced or eliminated by using facecoat or corecoat materials such ascarbon or graphite, high temperature oxides, refractory metals, halidesalts or the reactive metals themselves. These traditional containmentmethods usually are expensive, complex or even potentially hazardoussuch as when radioactive materials such as ThO₂ are used as the facecoator corecoat material. In addition, these traditional facecoat andcorecoat materials present the following technical limitations: (1) theyare often difficult to apply; (2) they often require controlledatmosphere firing and pre-heating; (3) even with these materials therecan still be a substantial risk of contamination from mold materials;and (4) the castings produced generally exhibit a substantial sectionthickness dependent reaction layer which must be removed, therebycausing difficulty in determining the as-cast part size necessary toproduce the finished part.

For a number of years, yttria (Y₂ O₃) has been investigated as apossible mold facecoat material because of its low reactivity withrespect to titanium. To make application of yttria economical,investigators have tried yttria-based slurries. Heretofore, however,investigators have been unsuccessful in using yttria-based slurries asmold facecoat materials in the fabrication of molds for casting reactivemetals.

For example in 1976, Schuyler et al. reported the results of tests usingfine particle yttria dispersed in colloidal potassium silicate solutionto which coarse yttria has been added as a mold facecoat material. D. R.Schuyler, et al., "Development of Titanium Alloy Casting Technology,"AFML-TR-76-80, August 1976, pp. 275-279. The molds made with thisfacecoat material were not satisfactory. Schuyler et al. reported that"the facecoat was not as smooth as normal for the standard foundrysystem. Pores and pits were present, and the stucco showed through inmany places." Schuyler et al. also tried a slurry containing yttria,titania and colloidal silica. Schuyler et al. found that with thissystem the facecoat surface was even more highly pitted.

Further unsuccessful attempts to use an yttria-based slurry as a moldfacecoat material were reported by Calvert in 1981. E. D. Calvert, "AnInvestment Mold for Titanium Casting," Bureau of Mines, RI8541, pp. 5-7,1981. Calvert reported that mold facecoat compositions comprising yttriapowder and aqueous colloidal silica binder resulted in slurries whichexhibited rapid and premature gellation and also resulted in moldsurfaces which exhibited a tendency to crack and spall during moldfiring. Similar results were obtained with yttria-based slurriescomprising yttria powder and a zirconium acetate binder. Calvert alsotried adding H₂ SO₄ to the yttria-based slurry but this caused porosityin the resulting titanium investment casting.

SUMMARY OF THE INVENTION

It is, therefore, a main object of the present invention to provide amold facecoat or corecoat material for the fabrication of molds for thecasting of reactive metals which overcomes the above-mentioneddrawbacks.

It is a more specific object of the present invention to provide anyttria-based slurry which can be used as a mold facecoat or corecoatmaterial for the fabrication of molds for casting reactive metals.

A further object of this invention is to provide a mold facecoat orcorecoat material for use in the fabrication of molds for castingreactive metals which reduces or eliminates reactivity between the moldand the reactive metal.

Another object of this invention is to provide an yttria based slurrymold facecoat which can be applied smoothly and evenly to the waxpattern used in the lost wax process for fabricating casting shells forcasting reactive metals.

A still further object of this invention is to provide an yttria-basedslurry corecoat which can be applied relatively smoothly and evenly to aceramic core in the fabrication of a casting core for casting hollowparts from reactive metals.

An additional object of this invention is to provide a method ofproducing high precision investment castings of reactive metals inlarge, small, or intricate shapes which were unobtainable with previousmold facecoats and corecoats.

A further object of this invention is to provide a method for producinghigh precision investment castings of reactive metals at a lower costthan previous techniques.

A still further object of this invention is to reduce the amount ofchemical milling required to produce high precision investment castingsof reactive metals.

Another object of this invention is to reduce or eliminate the surfacereaction layer (alpha-case) formed by the reaction between the mold andthe reactive metal in the investment casting of titanium and its alloys.Applicants also envision use of the present invention for a variety ofother foundry ceramic applications such as tundishes, filters, nozzlesand melting crucibles.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of instrumentalities and combinations particularly pointed out inthe appended claims.

To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventioncomprises a method of using an yttria-based slurry comprising a densegrain yttria powder and a non-aqueous-based binder as a mold facecoat orcorecoat in the fabrication of molds for casting reactive metals.

To further achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventioncomprises a method of fabricating a casting shell for casting reactivemetals comprising the steps of: preparing a pattern; dipping the patternin an yttria-based slurry comprising a dense grain yttria powder and anon-aqueous-based binder; forming a shell on the dipped pattern; dryingthe shell; removing the pattern; and firing the shell.

To further achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventioncomprises a method of making a casting core for fabricating a reactivemetal casting comprising the steps of: forming a removable ceramic core;coating the core with an yttria-based slurry comprising a dense grainyttria powder and a non-aqueous-based binder; and firing the coatedcore.

The foregoing and other objects, features and advantages of the presentinvention will be made more apparent from the following description ofthe preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention.

In accordance with the present invention, an yttria-based slurrycomprising a dense grain yttria powder and a non-aqueous-based binder isused as a mold facecoat or corecoat in the fabrication of molds forcasting reactive metals. For purposes of this invention, the termreactive metals refers to metals such as titanium and titanium alloyswhich have a high negative free energy of formation for the oxide,nitride, carbide or sulphide of the metal or component in the metal. Asembodied herein the reactive metals include, but are not limited to,titanium, titanium alloys, zirconium, zirconium alloys, aluminum-lithiumalloys and alloys containing significant amounts of yttrium, lanthanumor one of the other rare earth elements.

For purposes of the present invention, the dense grain yttria powder hasan apparent density greater than 4.60 grams per cubic centimeter (gm/cc)and preferably an apparent density greater than 4.90 gm/cc. The densegrain yttria powder can be formed by any number of conventionalprocesses such as sintering, fusing, crystallizing from solution orcalcining. In a preferred embodiment of the present invention, the densegrain yttria powder is a fused grain yttria powder having an apparentdensity of about 5.00 gm/cc. Preferably, the dense grain yttria powdercomprises between about 70% and 95% by weight of the yttria-basedslurry. More preferably, the dense grain yttria powder comprises betweenabout 75% and 90% by weight of the yttria-based slurry.

For purposes of the present invention, the non-aqueous-based binder ispreferably both a low temperature green strength and a high temperatureceramic binder. Preferably, the non-aqueous based binder is anorganometallic which includes a metal alkoxide, chelate, or containsmixed alkoxide-chelate ligands. Preferred organometallics useful in thepresent invention are silicon alkoxides and titanium alkoxide-chelates.Others which might be suitable are organometallics of zirconium,aluminum, yttrium, and the rare earth elements.

In a preferred embodiment of the present invention, thenon-aqueous-based binder includes the silicon alkoxide, ethyl silicate(also known as tetraethyl orthosilicate). Preferably, the silica (SiO₂)content of the binder is between about 4% and 18% by weight. Morepreferably the silica content is between about 8% and 13% by weight.Also preferably a hydrolyzed form of the ethyl silicate is used althoughthis is not necessary, especially if the binder system readilyhydrolyzes by taking up moisture from the air.

In another preferred embodiment of the present invention, thenon-aqueous-based binder includes a titanium alkoxide-chelate, such as atitanium-acetylacetonate-butoxide derivative. Preferably, the titania(TiO₂) content of the binder is between about 4% and 30% by weight. Morepreferably the titania content is between about 20% and 27% by weight.

For purposes of the present invention, the non-aqueous-based binder mayalso include additional additives or solvents to effect other desirablecharacteristics, such as to adjust the silica, titania or other metalcontent of the non-aqueous-based binder, to catalyze the binder, toadjust the hydrolysis level of the binder, to control the drying of thebinder; and/or to adjust the viscosity of the yttria-based slurry. In apreferred embodiment of the present invention wherein thenon-aqueous-based binder includes ethyl silicate, the binder alsoincludes a binder drying control additive such as propylene glycolmethyl ether (also known as monopropylene glycol monomethyl ether).

In accordance with a preferred embodiment of the present invention, theyttria-based slurry, comprising a dense grain yttria powder and atailored non-aqueous-based binder, is used to form a mold facecoat inthe fabrication of an investment casting shell by the "lost wax"process. As embodied herein, a pattern made of wax, plastic or anothersuitable material, such as frozen mercury or wood, having the shape ofthe desired casting (except for allowance for an overall shrinkagefactor) is prepared and dipped into the yttria-based slurry. Afterallowing the dipcoat layer to partially dry and/or cure, alternatelayers of ceramic stucco and dipcoat or alternate dipcoat layers areapplied over the original dipcoat until a shell of the desired thicknessis formed. The mold is allowed to dry thoroughly, and then, viaconventional techniques familiar to those skilled in the art, thepattern is removed by melting, dissolution and/or ignition.Subsequently, the mold is fired at a temperature above 1900° F., andpreferably at 2050°-2400° F., for a period in excess of 0.5 hours and ofpreferably 1-2 hours, in an oxidizing, inert or reducing atmosphere,preferably in an air atmosphere. Prior to the casing of metal, the moldmay be pre-heated to a temperature of about 200° F. or greater to ensurethat the mold is effectively free of moisture. In casting, the mold isfilled with molten metal with the assistance of gravity, pressure,centrifugal force, or other conventional techniques familiar to thoseskilled in the art. The metal is then allowed to cool. After cooling,the metal, shaped in the form of the original pattern, is removed andfinished by conventional methods familiar to those skilled in the art.

In accordance with another preferred embodiment of the presentinvention, an yttria-based slurry, comprising a dense grain yttriapowder and a non-aqueous-based binder, is employed as a corecoat in thefabrication of an investment casting core utilized in forming a hollowpart of a reactive metal casting. As embodied herein, a ceramic core,preferably a siliceously bonded metal oxide core, is suitably formulatedand fired. The core, in either a green (unfired) or fired state, is thencoated with an yttria-based slurry comprising a dense grain yttriapowder and a tailored non-aqueous-based binder. The slurry can bedeposited on the surface of the core by ordinary means, such as with anaerosol spray apparatus or by dipping. Cores coated with this slurry arepreferably fired at approximately 2050°-2400° F. for a period of atleast 1 hour in an air atmosphere. This firing may be performed eitheron the as-coated core or on the investment casting mold with coated corein place; the former being the preferred method. Mold fabrication, moldpre-heat, casting, mold knockout and metal finishing are essentially thesame as described above for the shell coating application. Core removalof conventional silica-based cores is accomplished by leachingtechniques employing a caustic agent as the leachant or by any otherappropriate method.

Preferred formulations for the yttria-based slurries used as moldfacecoats and mold corecoats in accordance with the present inventionare presented in Tables I and II, respectively. The yttria-based slurryused as a mold facecoat differs from the yttria-based slurry used as amold corecoat in that the latter includes more propylene glycol methylether to reduce the slurry viscosity.

                  TABLE I                                                         ______________________________________                                        PREFERRED FACECOAT FORMULATION                                                ______________________________________                                        Yttria Powder (Fused Grain, -325 mesh)                                                             2270         gm                                          Stauffer Silbond ® H-6 Prehydrolyzed Ethyl Silicate                                            243          ml                                          Dow Chemical Dowanol ® PM                                                                      107          ml                                          (Propylene Glycol Methyl Ether)                                               ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        PREFERRED CORE COATING FORMULATION                                            ______________________________________                                        Yttria Powder (Fused Grain, -325 mesh)                                                                   2270   gm                                          Stauffer Silbond ® H-6 Prehydrolyzed Ethyl Silicate                                                  243    ml                                          Dow Chemical Dowanol ® PM                                                                            187    ml                                          (Propylene Glycol Methyl Ether)                                               ______________________________________                                    

The Stauffer Silbond®H-6 prehydrolyzed ethyl silicate used in thepreferred formulations set forth in Tables I and II is a clear liquidhaving a density of 8.3 lbs./gal. at 68° F., an initial boiling point of172° F. (78° C.) at 1 atm., a freezing point below -70° F. (-57° C.), aflash point of 76° F. (24.5° C.) by TOC, a viscosity of 7 cps. at 20°C., a color of 100 APHA max., a specific gravity of 0.985-1.005 at15.6/15.6° C., an acidity of 0.050-0.060% max. (as HCl) and a silicacontent of 17.5-19.0% by wt. as SiO₂.

The Dow Chemical Dowanol®PM propylene glycol methyl ether used in thepreferred formulations set forth in Tables I and II is a solvent whichis completely soluble in water and has a specific gravity of 0.918-0.921at 25/25° C., an initial boiling point of 243° F. (117° C.) and adistillation point of 257° F. (125° C.) at 760 mm Hg, an acidity of 0.01wt.% max (as acetic acid), a water content of 0.25 wt.% max., a color of10 APHA max., a formula molecular weight of 90.1, a flash point of 89°F. (32° C.) by TCC, a refractive index of 1.404 at 68° F. (20° C.), aviscosity of 1.8 centistokes at 77° F. (25° C.), a vapor pressure of10.9 mm Hg at 77° F. (25° C.), a freezing point of -139° F. (-95° C.), asurface tension of 26.5 dynes/cm at 77° F. (25° C.) and an evaporationrate of 66 (BuAc=100).

The utility, suitability, and advantages of the yttria-based slurry ofthe present invention as a mold facecoat or corecoat in the fabricationof molds for casting reactive metals is illustrated by the comparativetesting described in the following examples:

EXAMPLE I

A facecoat evaluation was conducted on molds incorporating theyttria-based slurry composition of the present invention and 37 othervariations for investment casting step plates of Ti-6Al-4V alloy. Waxpatterns were fabricated in the form of the desired castings, withappropriate gating for molten metal feed. Individual patterns werecoated with the slurry formulations listed in Table III to form thefacecoat, or interior surface layer, on the mold for each pattern. Onsome patterns, two or three layers of the facecoat were utilized.Subsequent dipcoats on all molds were colloidal silica-bound zirconpowder formulations. Stucco material between each layer of dipcoat oneach mold was alumina grain. Eight layers of dipcoat/stucco wereapplied, followed by a cover dipcoat to minimize stucco spallationduring handling. Each step plate mold was dewaxed and then fired aslisted in Table III.

Prior to casting, the molds were assembled and pre-heated to 600° F. inair to minimize residual moisture. Under vacuum, molten Ti-6Al-4V wasfed into the molds which were rotated to generate a centrifugal forcefor increased metal fill. After allowing the molds to cool, the shellswere removed from the cast metal, and the gating was cut off.Metallographic examination of a cross-section through each step of thestep plate castings revealed a 48-92% (79% average) reduction inreaction layer (alpha-case) thickness due to using the yttria-basedslurry of the present invention, comprising a dense grain yttria powderand a non-aqueous-based binder (no. 38), compared to a prior artzirconia-based facecoat (no. 20). Results for each facecoat are given inTables III and IIIA. The fused grain yttria powder used in facecoat no.38 had a density of 5.00 gm/cc.

                                      TABLE                                       __________________________________________________________________________    FACECOAT EVALUATION - EXAMPLE I                                                                        No.            Avg Alpha-Case                                     Colloidal   of Firing      Reduction vs.                         ID                                                                              Powder     Binder      Dips                                                                             Cycle       Baseline (%).sup.                     __________________________________________________________________________    20                                                                              ZrO.sub.2 *                                                                              30% SiO.sub.2                                                                             1  1850° F./1h/air                                                                    Baseline                               1                                                                              TiN        "           "  1100° F./1h/Ar                                                                     54                                     2                                                                              "          "           "  1700° F./1h/Ar                                                                     54                                     3                                                                              "          "           "  1100° F./1h/Ar--5% H.sub.2                                                         Broke-Not Tested                       4                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         36                                     5                                                                              "          "           2  1100° F./1h/Ar                                                                     54                                     6                                                                              "          "           "  1700° F./1h/Ar                                                                     55                                     7                                                                              "          "           "  1100° F./1h/Ar--5% H.sub.2                                                         67                                     8                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         67                                     9                                                                              "          15% SiO.sub.2                                                                             1  1700° F./1h/Ar                                                                     18                                    10                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         71                                    11                                                                              "          "           2  1700° F./1h/Ar                                                                     58                                    12                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         59                                    13                                                                              TiC        30% SiO.sub.2                                                                             "  1100° F./1h/Ar                                                                     28                                    14                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         13                                    15                                                                              "          15% SiO.sub.2                                                                             "  1700° F./1h/Ar                                                                     26                                    16                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         15                                    17                                                                              "          20% Al.sub.2 O.sub.3                                                                      "  1700° F./1h/Ar                                                                     21                                    18                                                                              "          "           "  1950° F./1h/Ar--5% H.sub.2                                                         18                                    19                                                                              ZrO.sub.2 * + 3% TiO.sub.2                                                               30% SiO.sub.2                                                                             1  1850° F./1h/air                                                                     4                                    21                                                                              Er.sub.2 O.sub.3                                                                         "           "  1850° F./1h/air                                                                    32                                    22                                                                              "          20% ZrO.sub.2                                                                             "  1850° F./1h/air                                                                    Not Testable                          23                                                                              "          14% Y.sub.2 O.sub.3                                                                       "  1850° F./1h/air                                                                    Not Testable                          24                                                                              "          15% SiO.sub.2                                                                             2  1850° F./1h/air                                                                    46                                    25                                                                              Y.sub.2 O.sub.3                                                                          14% Y.sub.2 O.sub.3                                                                       "  2800° F./1h/air                                                                    56                                    26                                                                              Er.sub.2 O.sub.3 + 3% TiO.sub.2                                                          30% SiO.sub.2                                                                             1  1850° F./1h/air                                                                     7                                    27                                                                              TiN + 3% TiO.sub.2                                                                       "           "  1700° F./1h/Ar                                                                     42                                    28                                                                              ZrO.sub.2 */TiN (85/15)                                                                  "           "  1100° F./1h/Ar                                                                     14                                    29                                                                              ZrO.sub.2 */Er.sub.2 O.sub.3 (85/15)                                                     15% SiO.sub.2                                                                             "  1850° F./1h/air                                                                    43                                    30                                                                              ZrO.sub.2 */TiN (50/50)                                                                  30% SiO.sub.2                                                                             "  1100° F./1h/Ar                                                                     31                                    31                                                                              "          "           "  1700° F./1h/Ar                                                                     29                                    32                                                                              ZrO.sub.2 */Er.sub.2 O.sub.3 (50/50)                                                     "           "  1850° F./1h/air                                                                     4                                    33                                                                              "Y" Paint (Y.sub.2 O.sub.3)                                                              N/A         3  1850° F./1h/air                                                                    Not Testable                          34                                                                              TiN        20% ZrO.sub.2                                                                             1  1100° F./1h/Ar                                                                     54                                    35                                                                              "          "           "  1700° F./1h/Ar                                                                     54                                    36                                                                              ZrO.sub.2  "           "  2800° F./1h/air                                                                    -18 (increase)                        37                                                                              "          30% SiO.sub.2                                                                             2  1850° F./1h/vac                                                                    20                                    38                                                                              Y.sub.2 O.sub.3.sup.+                                                                    20% SiO.sub.2 Ethyl silicate                                                              "  1850° F./1h/air                                                                    79                                    __________________________________________________________________________     .sup. Average for 7 section thicknesses                                       *Contains 4.5 wt % CaO                                                        .sup.+ Fused grain                                                       

                  TABLE IIIA                                                      ______________________________________                                        FACECOAT EVALUATION - EXAMPLE I                                               ALPHA-CASE DEPTHS (MILS)                                                      AND AVG. ALPHA-CASE REDUCTIONS (%)                                                   Section Thickness                                                      ID           1/8"   1/4" 3/8" 1/2" 5/8" 3/4" 7/8" Avg.                        ______________________________________                                        20    Avg.   6.7    11.3 12.7 16.7 18.0 18.0 18.0                             (Base-                                                                              Max.    8     12   14   18   18   18   18                               line)                                                                          1    Avg.   1.3    3.3  6.7  8.0  9.3  10.7 10.7                                   Max.    2      4    8    8   10   12   12                                     %      81     71   47   52   48   41   41   54                                Red.                                                                     2    Avg.   2.0    4.0  5.3  6.0  8.7  11.3 12.7                                   Max.    2      4    6    6   10   14   14                                     %      70     65   58   64   52   37   30   54                                Red.                                                                     4    Avg.   2.0    6.0  9.3  12.0 12.0 13.3 14.7                                   Max.    4      8   10   12   12   16   16                                     %      70     47   27   28   33   26   18   36                                Red.                                                                     5    Avg.   2.0    4.0  6.7  8.0  8.7  10.0 10.0                                   Max.    2      6    8    8   10   10   10                                     %      70     65   47   52   52   44   44   54                                Red.                                                                     6    Avg.   4.0    5.3  8.0  4.7  6.0  7.3  8.0                                    Max.    4      4    8    6    6    8    8                                     %      40     53   37   72   67   59   56   55                                Red.                                                                     7    Avg.   1.0    3.3  4.0  4.7  7.3  8.0  8.0                                    Max.    2      4    4    6    8    8    8                                     %      85     71   68   72   59   56   56   67                                Red.                                                                     8    Avg.   1.0    3.3  4.0  6.0  6.7  7.3  8.0                                    Max.    2      4    4    6    8    8    8                                     %      85     71   68   64   63   59   56   67                                Red.                                                                     9    Avg.   5.3    8.0  10.7 13.3 14.0 16.0 16.7                                   Max.    6     10   12   14   16   18   18                                     %      21     29   16   20   22   11    7   18                                Red.                                                                    10    Avg.    .5     .5  4.0  4.7  7.3  8.0  8.0                                    Max.    1      1    4    6    8    8   10                                     %      93     96   68   72   59   56   56   71                                Red.                                                                    11    Avg.   1.0    4.0  4.7  6.0  9.3  10.0 11.3                                   Max.    2      4    6    6   10   10   12                                     %      85     64   63   64   48   44   37   58                                Red.                                                                    12    Avg.   3.3    4.0  4.0  6.0  6.7  8.7  9.3                                    Max.    4      4    4    6    8   10   12                                     %      51     65   68   64   63   52   48   59                                Red.                                                                    13    Avg.   4.7    7.3  10.0 12.0 14.0 13.3 12.7                                   Max.    6      8   10   12   14   14   14                                     %      30     35   21   28   22   26   29   28                                Red.                                                                    14    Avg.    .6    10.0 11.3 13.3 14.7 16.0 16.0                                   Max.    8     12   12   14   16   16   16                                     %      10     12   11   20   18   11   11   13                                Red.                                                                    15    Avg.    .2    7.3  10.7 12.7 14.0 15.3 18.0                                   Max.    2     10   12   14   14   16   22                                     %      70     35   15   24   22   15    0   26                                Red.                                                                    16    Avg.   5.3    9.3  10.7 14.7 14.7 16.0 16.0                                   Max.    6     10   12   16   16   16   16                                     %      21     18   16   12   18   11   11   15                                Red.                                                                    17    Avg.   8.0    7.0  7.0  13   12   15   16                                     Max.   10     10   12   16   37   28   36                                     %      -19    38   45   22   33   17   11   21                                Red.                                                                    18    Avg.   7.3    7.3  8.0  12.0 16.0 16.0 15.3                                   Max.    8      8    8   16   16   16   16                                     %      -9     35   37   28   11   11   15   18                                Red.                                                                    19    Avg.   6.7    10.7 13.3 14.0 16.7 18.0 16.7                                   Max.    8     12   14   14   18   18   18                                     %       0      5   -5   16    7    0    7    4                                Red.                                                                    21    Avg.   6.0    7.3  10.0 9.3  10.7 12.0 10.7                                   Max.    6      8   10   10   12   12   12                                     %      10     35   21   44   41   33   41   32                                Red.                                                                    24    Avg.   4.0    4.0  7.3  9.3  8.7  10.0 11.3                                   Max.    4      4    8   10   10   10   12                                     %      40     65   42   44   52   44   37   46                                Red.                                                                    25    Avg.   4.0    3.3  5.3  6.7  7.3  7.3  10.7                                   Max.    4      4    6    8    8    8   12                                     %      40     71   58   60   59   59   40   56                                Red.                                                                    26    Avg.   6.7    11.3 13.3 15.3 14.7 14.7 16.0                                   Max.    8     12   16   16   16   16   18                                     %       0      0   -5    8   18   18   11    7                                Red.                                                                    27    Avg.   4.7    5.3  7.3  8.7  8.7  11.3 12.7                                   Max.    6      6    8   10   10   12   14                                     %      30     53   42   48   52   37   29   42                                Red.                                                                    28    Avg.   7.3    8.7  10.7 14.0 14.0 15.3 16.0                                   Max.    8     10   12   16   14   16   16                                     %      -9     23   16   16   22   15   11   14                                Red.                                                                    29    Avg.   4.0    6.7  6.7  8.0  9.3  11.3 12.0                                   Max.    4      8    8   10   10   12   12                                     %      40     41   47   52   48   37   33   43                                Red.                                                                    30    Avg.   4.0    6.7  10.7 10.7 12.0 14.0 12.7                                   Max.    4      8   12   12   12   14   14                                     %      40     41   16   36   33   22   29   31                                Red.                                                                    31    Avg.   6.7    6.7  8.7  10.0 12.0 12.7 14.0                                   Max.    8      6   10   10   12   14   14                                     %       0     41   32   40   33   33   22   29                                Red.                                                                    32    Avg.   6.7    10.7 14.0 14.0 16.0 16.7 18.0                                   Max.    8     14   14   14   16   18   18                                     %       0      5   -10  16   11    7    0    4                                Red.                                                                    34    Avg.    .5    4.0  6.7  8.0  8.7  10.0 13.3                                   Max.    1      6    8    8   10   10   16                                     %      92     65   47   52   52   44   26   54                                Red.                                                                    35    Avg.   1.3    5.3  6.0  8.3  10.0 10.0 10.0                                   Max.    2      6    6   10   10   10   10                                     %      81     53   53   50   44   44   52   54                                Red.                                                                    36    Avg.   6.7    10.0 14.7 17.7 23.3 25.3 25.7                                   Max.    8     10   16   20   26   26   26                                     %       0     12   -16  -6   -29  -41  -43  -18                               Red.                                                                    37    Avg.   6.0    7.3  10.0 12.7 14.7 14.7 15.3                                   Max.    6      8   12   14   16   16   16                                     %      10     35   21   24   18   18   15   20                                Red.                                                                    38    Avg.   1.0    1.0  1.0  2.0  4.0  6.0  9.3                                    Max.    1      1    1    2    4    6   12                                     %      85     91   92   88   78   67   48   79                                Red.                                                                    ______________________________________                                    

EXAMPLE II

A second trial was performed to evaluate 26 facecoat systems, including4 yttria-based facecoat systems of the present invention (nos. 12, 16,17 and 18) for investment casting step plates of Ti-6Al-4V alloy. Thesystems tested are listed in Table IV. Systems 16, 17 and 18 used azircon powder/ethyl silicate binder back-up dip in place of the standardzircon powder/colloidal silica bound formulation. The trial wasconducted in the same manner as in Example I. Results for each facecoatare given in Tables IV and IVA. Prior art zirconia-based facecoat (no.9) was used as a baseline. The fused grain yttria powder used infacecoat nos. 12 and 14-18 had a density of 5.00 gm/cc. The unfusedgrain yttria used in facecoat no. 33 had a density of 4.60 gm/cc.

                                      TABLE IV                                    __________________________________________________________________________    FACECOAT EVALUATION - EXAMPLE II                                                                                       Avg.                                                                          Alpha-Case                                                                    Reduction vs.                        ID                                                                              Powder  Binder*                                                                              Firing      Comments    Baseline (%).sup.                    __________________________________________________________________________     9                                                                              ZrO.sub.2                                                                             30% SiO.sub.2 Sol                                                                    1850° F./1 h/air Baseline                             10                                                                              "       Ethyl Silicate                                                                       "                       -6                                   26                                                                              "       30% SiO.sub.2 Sol                                                                    1850° F./4 h/vac  6                                   31                                                                              "       12% TiO.sub.2 Sol                                                                    1850° F./1 h/air  4                                    1                                                                              TiN     30% SiO.sub.2 Sol                                                                    1100° F./1 h/Ar  52                                    2                                                                              "       "      1100° F./1 h/Ar--5% H.sub.2                                                                    49                                    4                                                                              "       "      1950° F./1 h/Ar--5% H.sub.2                                                                    60                                    5                                                                              "       Ethyl silicate                                                                       1100° F./1 h/Ar  55                                    6                                                                              "       "      1950° F./1 h/Ar--5% H.sub.2                                                                    59                                   25                                                                              "       30% SiO.sub.2 Sol                                                                    1100° F./1 h/Ar                                                                    Dip Pot Application                                                                       44                                   32                                                                              "       12% TiO.sub.2 Sol                                                                    "                       38                                   12                                                                              Y.sub.2 O.sub.3                                                                       Ethyl silicate                                                                       1850° F./1 h/air 57                                   14                                                                              "       30% SiO.sub.2 Sol                                                                    "                       18                                   15                                                                              "       "      2700° F./1 h/air  4                                   16                                                                              "       Ethyl silicate                                                                       1850° F./1 h/air                                                                   ZrSiO.sub.4 /Et. sil. Backup                                                              76                                   17                                                                              "       "      2400° F./1 h/air                                                                   "           81                                   18                                                                              "       "      2700° F./1 h/air                                                                   "           70                                   33                                                                              Y.sub.2 O.sub.3 **                                                                    "      1850° F./1 h/air 33                                   27                                                                              ThO.sub.2                                                                             "      "                       17                                   28                                                                              "       15% SiO.sub.2 Sol                                                                    "                        9                                   29                                                                              "       30% SiO.sub.2 Sol                                                                    "                       15                                   19                                                                              Er.sub.2 O.sub.3                                                                      "      "                       31                                   21                                                                              LaOF    "      "                       -2                                   22                                                                              CeO.sub.2 + CeF.sub.3                                                                 "      "                       -1                                   23                                                                              YOF     "      "                       -7                                   24                                                                              NdOF    "      "                        8                                   __________________________________________________________________________       Averaged over seven section thicknesses (negative value indicates           increase)                                                                     *All ethylsilicate binders contain approximately 13 wt %                       Fused grain                                                                  **Unfused grain                                                               All facecoats were single dip                                            

                  TABLE IVA                                                       ______________________________________                                        FACECOAT EVALUATION - EXAMPLE II                                              ALPHA-CASE DEPTHS (MILS)                                                      AND AVG. ALPHA-CASE REDUCTIONS (%)                                                   Section Thickness                                                      ID           1/8"   1/4" 3/8" 1/2" 5/8" 3/4" 7/8" Avg.                        ______________________________________                                         9    Avg.   3.5    6.6  9.9  12.8 14.8 15.6 16.8                             (Base-                                                                              Max.   7.2    11.2 16.0 16.0 18.0 18.0 20.0                             line                                                                          10    Avg.   3.7    7.3  11.6 13.3 15.2 16.4 16.0                                   Max.   4.6    8.8  14.0 16.0 18.0 19.2 18.0                                   %      -5     -10  -17  -4   -2   -6    5   -6                                Red.                                                                    26    Avg.   2.5    7.0  9.6  12.0 14.6 15.4 14.6                                   Max.   4.0    10.8 10.8 16.0 18.0 20.0 16.0                                   %      28     -1    3    6    1    1   13    6                                Red.                                                                    31    Avg.   4.0    5.5  8.9  10.4 15.0 14.6 18.0                                   Max.   4.0    6.0  12.0 14.0 22.0 18.0 24.0                                   %      -14    17   10   18   -1    6   -7    4                                Red.                                                                     1    Avg.   1.3    2.3  4.4  6.0  8.3  9.5  8.5                                    Max.   2.4    4.0  5.6  6.4  10.0 12.8 10.0                                   %      62     65   55   53   44   39   49   52                                Red.                                                                     2    Avg.   1.0    3.4  4.7  7.5  7.3  9.3  10.2                                   Max.   2.0    4.0  6.4  9.6  8.0  10.0 12.0                                   %      72     48   52   42   50   40   39   49                                Red.                                                                     4    Avg.   1.0    2.6  4.4  5.6  6.4  6.1  7.2                                    Max.   2.0    4.8  5.2  8.0  8.8  6.4  8.4                                    %      72     61   55   56   56   61   57   60                                Red.                                                                     5    Avg.    .7    2.4  4.8  6.3  8.4  8.0  8.4                                    Max.   2.0    4.0  6.0  8.0  10.0 10.0 10.0                                   %      79     63   52   51   44   49   50   55                                Red.                                                                     6    Avg.    .8    2.0  4.2  7.0  7.2  7.2  6.3                                    Max.   1.6    3.2  4.4  8.0  8.0  8.0  8.0                                    %      75     69   58   46   52   54   62   59                                Red.                                                                    25    Avg.    .7    2.7  6.2  8.0  9.9  12.6 9.9                                    Max.   2.0    4.0  8.0  12.0 12.8 18.0 12.0                                   %      79     59   38   37   33   19   41   44                                Red.                                                                    32    Avg.   1.3    4.0  6.2  7.4  10.3 11.0 13.3                                   Max.   3.2    7.2  8.0  10.0 12.0 12.0 24.0                                   %      64     40   37   42   30   29   21   38                                Red.                                                                    12    Avg.   1.7    2.0  3.0  5.3  7.8  8.6  7.0                                    Max.   4.0    4.0  7.2  12.0 14.0 16.0 10.0                                   %      51     69   70   58   48   44   58   57                                Red.                                                                    14    Avg.   4.1    6.0  7.2  9.5  10.3 12.7 10.6                                   Max.   8.0    10.0 12.0 12.0 12.0 16.4 12.0                                   %      -18     9   27   26   30   18   37   18                                Red.                                                                    15    Avg.   3.9    7.4  9.9  12.6 11.6 14.6 12.3                                   Max.   6.0    11.2 14.0 14.0 12.0 16.0 14.0                                   %      -12    -12   0    1   22    6   27    4                                Red.                                                                    16    Avg.    .8    1.3  2.2  2.4  4.0  3.8  5.3                                    Max.   1.0    1.6  4.0  2.5  6.0  4.9  10.0                                   %      76     79   77   81   72   76   68   76                                Red.                                                                    17    Avg.    .8    1.1  1.5  1.5  2.4  3.4  4.6                                    Max.   1.2    2.0  2.0  2.0  4.0  6.0  6.0                                    %      75     83   84   88   84   78   72   81                                Red.                                                                    18    Avg.    .9    1.6  2.0  5.0  4.3  6.0  5.6                                    Max.   2.0    4.0  2.8  18.0 12.0 20.0 9.2                                    %      73     76   80   61   71   61   66   70                                Red.                                                                    33    Avg.   1.5    4.2  6.8  10.0 11.2 11.0 12.2                                   Max.   2.0    8.0  10.0 10.0 14.0 16.0 14.0                                   %      58     36   31   22   24   29   28   33                                Red.                                                                    27    Avg.   2.4    5.9  8.5  11.3 12.6 13.3 12.9                                   Max.   4.0    10.0 12.0 14.0 18.0 20.0 18.0                                   %      30     10   14   12   15   14   23   17                                Red.                                                                    28    Avg.   3.1    6.4  10.3 12.0 12.6 14.0 13.3                                   Max.   6.0    8.0  16.0 18.0 20.0 16.0 16.0                                   %      11      2   -4    6   15   10   21    9                                Red.                                                                    29    Avg.   2.2    5.6  8.9  11.6 13.8 14.2 12.8                                   Max.   4.0    10.0 14.0 16.0 18.0 18.0 16.0                                   %      35     14   10    9    7    9   23   15                                Red.                                                                    19    Avg.   2.0    3.8  6.6  9.3  11.6 12.3 11.6                                   Max.   3.2    5.6  8.0  12.0 16.8 18.0 12.6                                   %      43     42   33   27   22   21   31   31                                Red.                                                                    21    Avg.   2.8    7.1  10.8 13.8 15.4 17.3 15.3                                   Max.   4.0    10.0 16.0 16.8 20.0 20.0 16.0                                   %      18     -8   -8   -8   -4   -11   9   -2                                Red.                                                                    22    Avg.   3.8    8.2  10.1 11.8 13.3 15.5 14.6                                   Max.   6.0    12.0 12.0 16.0 16.0 19.2 16.0                                   %      -8     -25  -2    8   10    0   13   -1                                Red.                                                                    23    Avg.   3.8    8.2  9.3  14.0 16.0 17.6 15.6                                   Max.   6.0    12.0 14.0 20.0 20.0 22.0 18.0                                   %      -8     -25   6   -9   -8   -13   7   -7                                Red.                                                                    24    Avg.   3.3    6.8  9.2  10.5 13.0 15.4 14.8                                   Max.   6.8    10.0 12.0 14.0 18.0 18.0 16.0                                   %       5     -3    7   18   12    1   12    8                                Red.                                                                    ______________________________________                                    

EXAMPLE III

A third trial was performed to evaluate 23 facecoat systems, including18 yttria-based facecoats of the present invention (facecoat nos. 2-12,15, 17, 18, 21-23 and 33), for investment casting step plates ofTi-6Al-4V alloy. The systems tested are listed in Table V. Processingand materials modifications are noted in Table V. The trial wasconducted in the same manner as in Example I. Results for each facecoatare reported in Tables V and VA. Once again a prior art zirconia-basedfacecoat was used as a baseline. The fused grain yttria powder used inthe facecoat nos. 2-12, 15, 17, 18, 21-23 and 33 had a density of 5.00gm/cc.

    __________________________________________________________________________    FACECOAT EVALUATION - EXAMPLE III                                                                No.                      Avg.                                                 Of                       Alpha-Case Reduction              ID                                                                              Powder  Binder   Dips                                                                             Firing  Comments      vs. Baseline                      __________________________________________________________________________                                                (%)                                1                                                                              ZrO.sub.2                                                                             30% SiO.sub.2 Sol                                                                      1  1850° F./1 h/air                                                                             Baseline                          33                                                                              Y.sub.2 O.sub.3 *                                                                      4% SiO.sub.2 Eth Sil                                                                  "  2200° F./1 h/air                                                               Varying Binder SiO.sub.2                                                                    63                                 2                                                                              "        8% SiO.sub.2 Eth Sil                                                                  "  "       "             74                                18                                                                              "       13% SiO.sub.2 Eth Sil                                                                  "  "       "             72                                 4                                                                              "       18% SiO.sub.2 Eth Sil                                                                  "  "       "             62                                 5                                                                              "       13% SiO.sub.2 Eth Sil                                                                  "  1850° F./ 1 h/air                                                              Varying Firing Temp                                                                         55                                 6                                                                              "       "        "  2000° F./1 h/air                                                               "             46                                 7                                                                              "       "        "  2100° F./1 h/air                                                               "             72                                 8                                                                              "       "        "  2300° F./1 h/air                                                               "             71                                 9                                                                              "       "        "  2400° F./1 h/air                                                               "             72                                10                                                                              "       "        "  1850° F./1 h/air                                                               Eth Sil Bonded Backup Dips                                                                  50                                11                                                                              "       "        "  2200° F./1 h/air                                                               "             65                                12                                                                              "       "        "  2400° F./1 h/air                                                               "             60                                15                                                                              "       "        "  2200° F./1 h/air                                                               Air Classified (+1 micron)                                                                  80                                16                                                                              ZrO.sub.2                                                                             30% SiO.sub.2 Sol                                                                      2  1850° F./1 h/air                                                               Double Facecoat Dips                                                                        -7   (increase)                   17                                                                              Y.sub.2 O.sub.3 *                                                                      8% SiO.sub.2 Eth Sil                                                                  "  2200° F./1 h/air                                                               "             75                                 3                                                                              "       13% SiO.sub.2 Eth Sil                                                                  "  "       "             63                                20                                                                              ZrO.sub.2                                                                             "        1  "       100 w/oZrO.sub.2                                                                            2                                 21                                                                              ZrO.sub.2 + Y.sub.2 O.sub.3 *                                                         "        "  "       75 w/oZrO.sub.2 + 25 w/oY.sub.2 O.sub.3                                                     12                                22                                                                              "       "        "  "       50 w/oZrO.sub.2 + 50 w/oY.sub.2 O.sub.3                                                     26                                23                                                                              "       "        "  "       25 w/oZrO.sub.2 + 75 w/oY.sub.2 O.sub.3                                                     44                                29                                                                              MgO     "        "  1850° F./1 h/air                                                               Fused Grain Powder                                                                          21                                30                                                                              "       "        "  2200° F./1 h/air                                                               "             15                                __________________________________________________________________________     *-325 fused grain Y.sub.2 O.sub.3                                              Average reduction for 7 section thicknesses                             

                  TABLE VA                                                        ______________________________________                                        FACECOAT EVALUATION - EXAMPLE III                                             ALPHA-CASE DEPTHS (MILS)                                                      AND AVG. ALPHA-CASE REDUCTIONS (%)                                                   Section Thickness                                                      ID           1/8"   1/4" 3/8" 1/2" 5/8" 3/4" 7/8" Avg.                        ______________________________________                                         1    Avg.   5.8    9.1  14.3 15.8 18.5 18.6 17.4                             (Base-                                                                              Max.   7.5    12.5 16.0 18.0 22.0 21.0 19.0                             line)                                                                         33    Avg.   1.5    2.8  3.5  6.3  6.5  7.8  10.0                                   Max.   3.5    6.5  6.0  9.0  9.5  11.5 12.5                                   %      74     69   75   60   64   58   42   63                                Red.                                                                     2    Avg.    .5    1.0  2.4  3.8  5.3  6.5  9.2                                    Max.   1.0    1.5  3.0  4.0  6.5  9.0  11.5                                   %      90     89   83   76   71   65   47   74                                Red.                                                                    18    Avg.    .8    2.0  3.5  5.4  5.3  6.1  6.5                                    Max    1.0    2.0  3.5  7.0  7.0  10.0 9.0                                    %      86     77   75   66   71   67   63   72                                Red.                                                                     4    Avg.   1.5    3.1  4.5  5.5  8.0  8.5  9.0                                    Max.   2.5    8.0  8.5  10.0 9.0  13.0 12.5                                   %      73     65   69   65   57   54   48   62                                Red.                                                                     5    Avg.   2.2    4.3  5.2  7.1  7.9  8.9  9.8                                    Max.   6.5    8.0  9.0  11.5 11.5 12.5 14.0                                   %      61     53   64   55   57   52   44   55                                Red.                                                                     6    Avg.   2.6    4.5  8.0  9.9  10.0 9.3  10.8                                   Max.   6.0    7.0  11.0 11.5 12.0 13.0 15.0                                   %      54     51   44   37   46   50   38   46                                Red.                                                                     7    Avg.    .9    1.8  4.0  4.7  4.0  5.0  8.8                                    Max.   2.0    3.5  7.0  7.0  7.5  7.0  10.0                                   %      84     80   72   70   78   73   49   72                                Red.                                                                     8    Avg.   1.3    2.4  3.5  4.5  4.5  4.3  8.6                                    Max.   2.5    4.0  5.0  6.0  6.5  7.0  11.5                                   %      77     74   75   71   75   77   50   71                                Red.                                                                     9    Avg.   1.2    2.2  3.5  4.0  4.9  5.8  7.2                                    Max.   3.0    3.5  5.0  6.0  7.5  8.5  8.5                                    %      78     75   75   74   73   69   58   72                                Red.                                                                    10    Avg.   3.4    4.6  7.0  7.9  8.6  9.3  8.3                                    Max.   8.0    9.0  11.5 12.0 18.0 16.0 12.0                                   %      41     49   51   50   53   50   52   50                                Red.                                                                    11    Avg.   1.0    2.9  5.4  7.0  7.5  6.6  6.5                                    Max.   1.5    8.0  8.0  14.5 12.5 11.0 9.0                                    %      82     68   62   56   60   64   62   65                                Red.                                                                    12    Avg.   1.3    2.5  5.1  7.2  9.3  8.3  9.5                                    Max.   4.5    5.0  9.0  9.5  14.0 11.0 11.5                                   %      77     72   64   54   50   55   45   60                                Red.                                                                    15    Avg.    .6    1.6  3.3  3.1  4.0  3.7  5.3                                    Max.   1.0    2.5  4.5  4.0  5.5  5.0  6.0                                    %      88     83   77   80   78   80   69   80                                Red.                                                                    16    Avg.   5.8    10.4 14.7 17.4 19.0 20.7 18.9                                   Max.   8.5    12.5 17.5 22.0 24.0 23.0 21.0                                   %      -1     -14  -3   -10  -3   -11  -9   -7                                Red.                                                                    17    Avg.    .4    1.0  2.8  3.9  4.6  5.7  9.1                                    Max.    .5    2.0  3.5  5.0  6.0  8.0  11.5                                   %      93     88   80   75   75   69   47   75                                Red.                                                                     3    Avg.   1.0    2.7  4.4  4.7  6.5  10.0 10.9                                   Max.   2.5    5.5  8.0  7.0  8.0  15.0 13.0                                   %      81     70   69   70   65   46   37   63                                Red.                                                                    20    Avg.   5.1    8.9  12.3 16.2 19.1 18.3 18.8                                   Max.   7.5    11.5 15.0 19.0 22.0 20.0 19.7                                   %      11      3   14   -3   -4    2   -8    2                                Red.                                                                    21    Avg.   6.0    8.1  11.5 13.8 16.1 16.5 14.7                                   Max.   7.0    11.0 15.0 17.0 18.0 18.0 15.0                                   %      -3     11   20   13   13   12   16   12                                Red.                                                                    22    Avg.   4.8    7.3  8.5  11.7 13.1 14.2 13.0                                   Max.   6.0    8.5  11.0 15.0 14.5 20.0 15.5                                   %      17     20   40   26   29   24   25   26                                Red.                                                                    23    Avg.   3.0    4.9  6.6  9.3  10.0 11.2 11.0                                   Max.   5.0    6.0  9.0  12.5 11.5 14.0 15.0                                   %      48     46   53   41   46   40   36   44                                Red.                                                                    29    Avg.   4.2    7.4  9.4  12.6 13.5 15.0 17.5                                   Max.   6.0    11.0 11.5 16.0 16.0 16.0 23.0                                   %      27     19   34   20   27   20    0   21                                Red.                                                                    30    Avg.   4.5    8.7  11.5 14.5 14.5 16.5 14.1                                   Max.   7.0    11.0 14.0 17.5 16.0 19.0 16.0                                   %      23      4   19    8   22   11   19   15                                Red.                                                                    ______________________________________                                    

EXAMPLE IV

A fourth trial was performed wherein 17 hollow step wedges were cast inTi-6Al-4V. The systems tested, along with materials and processconfigurations, are listed in Table VI. The systems tested included 8yttria-based corecoats of the present invention (corecoat nos. 6-13).After each core was coated, (and fired, if indicated), each core wasincorporated into a step wedge wax pattern. The wax patternssubsequently were incorporated into individual shells, utilizing theprior art zirconia powder/colloidal silica binder facecoat for allspecimens. The remainder of the trial was conducted in the same manneras Example I. Results for each core/corecoating system are given inTables VI and VIA. Again a prior art zirconia-based corecoat was used asa baseline. The yttria used in the corecoat nos. 6-13 and 22 was fusedgrain yttria powder having a density of 5.00 gm/cc.

                                      TABLE VI                                    __________________________________________________________________________    CORE COATING EVALUATION - EXAMPLE IV                                                                                     Avg. Alpha-Case                                Coating                        Reduction vs.                      ID                                                                              Core Coating.sup.++                                                                     Application.sup.                                                                     Coating Fire                                                                              Mold Fire   Baseline (%)*                      __________________________________________________________________________     2                                                                              ZrO.sub.2 /30% SiO.sub.2 sol                                                            PS     1850° F./1 h/air                                                                   1850° F./1 h/air                                                                   Baseline                            4                                                                              ZrO.sub.2 CVD    None        "           -14                                 5                                                                              HfO.sub.2 "      "           "           12                                  6                                                                              Y.sub.2 O.sub.3 /eth sil                                                                Dipping                                                                              1850° F./1 h/air                                                                   "           87                                  7                                                                              "         PS     "           "           55                                  8                                                                              "         "      2050° F./1 h/air                                                                   "           89                                  9                                                                              "         "      2250° F./1 h/air                                                                   "           91                                 10                                                                              "         "      2450° F./1 h/air                                                                   "           86                                 11                                                                              "         PS (2 coats)                                                                         2250° F./1 h/air                                                                   "           87                                 12                                                                              "         PS     "           2250° F./1 h/air                                                                   34                                 13                                                                              "         "      None        "           70                                 14                                                                              TiN/30% SiO.sub.2 sol                                                                   Dipping                                                                              1850° F./1 h/Ar--5% H.sub.2                                                        1850° F./1 h/Ar--5%                                                                33sub.2                            15                                                                              "         PS     "           "           22                                 17                                                                              Nd.sub.2 O.sub.3 /eth sil                                                               Dipping                                                                              2250° F./1 h/air                                                                   1850° F./1 h/air                                                                   -31                                18                                                                              MgO/eth sil                                                                             "      "           "            2                                 22                                                                              Y.sub.2 O.sub.3                                                                         APS    "           "           98                                 23                                                                              ZrO.sub.2 --7w/oY.sub.2 O.sub.3                                                         "      "           "           28                                 __________________________________________________________________________     *Average for 7 section thicknesses (Negative value indicates alphacase        increase)                                                                     .sup. PS: Aerosol spray                                                       CVD: Chemical Vapor Deposition                                                APS: Air Plasma Spray                                                         .sup.++ All ethyl silicate binders contain approximately 13 wt % siO.sub.

                                      TABLE VIA                                   __________________________________________________________________________    CORE COATING EVALUATION - EXAMPLE IV                                          ALPHA-CASE DEPTHS (MILS)                                                      AND AVG. ALPHA-CASE REDUCTIONS (%)                                                       Section Thickness                                                  ID         1/16"                                                                            1/8"                                                                             1/4"                                                                             3/8"                                                                             1/2"                                                                             5/8"                                                                             3/4"                                                                             7/8"                                                                             Avg.                                       __________________________________________________________________________    2     Avg. 2.6                                                                              5.3                                                                              9.1                                                                              12.0                                                                             13.8                                                                             14.8                                                                             15.0                                                                             15.8                                          (Baseline)                                                                          Max. 5.0                                                                              6.0                                                                              12.0                                                                             16.0                                                                             17.0                                                                             17.0                                                                             17.0                                                                             19.0                                          4     Avg. 3.0                                                                              5.8                                                                              11.0                                                                             13.5                                                                             15.1                                                                             16.5                                                                             18.0                                                                             19.3                                                Max. 4.0                                                                              6.0                                                                              12.0                                                                             15.0                                                                             16.0                                                                             17.0                                                                             19.0                                                                             20.0                                                % Red.                                                                             -13                                                                              -9 -16                                                                              -12                                                                              -10                                                                              -11                                                                              -20                                                                              -22                                                                              -14                                        5     Avg. 1.6                                                                              4.1                                                                              7.6                                                                              11.0                                                                             13.0                                                                             13.8                                                                             14.8                                                                             16.1                                                Max. 4.0                                                                              6.0                                                                              10.0                                                                             13.0                                                                             14.0                                                                             17.0                                                                             17.0                                                                             17.0                                                % Red.                                                                             37 22 16 8  6  7  1  -2 12                                         6     Avg. 0.0                                                                              0.6                                                                              0.5                                                                              1.5                                                                              2.8                                                                              2.1                                                                              2.5                                                                              2.6                                                 Max. 0.0                                                                              2.0                                                                              2.0                                                                              3.0                                                                              6.0                                                                              6.0                                                                              6.0                                                                              6.0                                                 % Red.                                                                             100                                                                              87 94 87 80 85 83 83 87                                         7     Avg. 1.6                                                                              0.8                                                                              1.6                                                                              3.3                                                                              7.0                                                                              8.3                                                                              9.0                                                                              10.5                                                Max. 3.0                                                                              1.0                                                                              2.0                                                                              5.0                                                                              11.0                                                                             13.0                                                                             14.0                                                                             13.0                                                % Red.                                                                             37 84 82 72 49 44 40 34 55                                         8     Avg. 0.8                                                                              0.5                                                                              0.5                                                                              0.8                                                                              1.3                                                                              1.1                                                                              1.0                                                                              1.2                                                 Max. 3.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              2.0                                                                              2.0                                                                              1.0                                                                              2.0                                                 % Red.                                                                             69 91 94 93 90 92 93 92 89                                         9     Avg. 0.5                                                                              0.5                                                                              0.5                                                                              0.6                                                                              1.1                                                                              1.1                                                                              0.8                                                                              1.6                                                 Max. 1.0                                                                              1.0                                                                              1.0                                                                              2.0                                                                              4.0                                                                              4.0                                                                              2.0                                                                              4.0                                                 % Red.                                                                             81 91 94 94 92 92 94 90 91                                         10    Avg. 0.0                                                                              0.0                                                                              0.5                                                                              2.0                                                                              2.0                                                                              1.5                                                                              2.0                                                                              8.0                                                 Max. 0.0                                                                              0.0                                                                              1.0                                                                              4.0                                                                              3.0                                                                              2.0                                                                              3.0                                                                              20.0                                                % Red.                                                                             100                                                                              100                                                                              94 83 86 90 87 49 86                                         11    Avg. 0.0                                                                              0.5                                                                              1.1                                                                              2.0                                                                              2.0                                                                              2.1                                                                              1.6                                                                              3.7                                                 Max. 0.0                                                                              1.0                                                                              2.0                                                                              4.0                                                                              3.0                                                                              5.0                                                                              2.0                                                                              5.0                                                 % Red.                                                                             100                                                                              91 87 83 86 85 89 77 87                                         12    Avg. 2.1                                                                              3.1                                                                              4.1                                                                              7.0                                                                              7.1                                                                              9.8                                                                              10.0                                                                             15.1                                                Max. 3.0                                                                              4.0                                                                              7.0                                                                              14.0                                                                             10.0                                                                             12.0                                                                             12.0                                                                             19.0                                                % Red.                                                                             19 41 54 41 48 34 33 4  34                                         13    Avg. 0.8                                                                              0.8                                                                              1.0                                                                              2.5                                                                              4.5                                                                              5.8                                                                              5.1                                                                              8.8                                                 Max. 2.0                                                                              2.0                                                                              2.0                                                                              4.0                                                                              7.0                                                                              9.0                                                                              9.0                                                                              15.0                                                % Red.                                                                             69 84 89 79 67 61 66 44 70                                         14    Avg. 3.1                                                                              2.6                                                                              4.8                                                                              7.0                                                                              8.6                                                                              8.8                                                                              10.3                                                                             10.3                                                Max. 5.0                                                                              4.0                                                                              7.0                                                                              8.0                                                                              10.0                                                                             10.0                                                                             14.0                                                                             12.0                                                % Red.                                                                             -18                                                                              50 47 42 37 40 31 35 33                                         15    Avg. 1.6                                                                              5.0                                                                              7.5                                                                              9.2                                                                              9.6                                                                              11.6                                                                             11.0                                                                             13.3                                                Max. 3.0                                                                              7.0                                                                              10.0                                                                             12.0                                                                             10.0                                                                             13.0                                                                             13.0                                                                             15.0                                                % Red.                                                                             37 6  18 24 30 21 27 16 22                                         17    Avg. 5.0                                                                              7.5                                                                              11.3                                                                             14.5                                                                             15.1                                                                             17.1                                                                             19.1                                                                             19.0                                                Max. 6.0                                                                              9.0                                                                              12.0                                                                             16.0                                                                             16.0                                                                             19.0                                                                             22.0                                                                             21.0                                                % Red.                                                                             -88                                                                              -42                                                                              -24                                                                              -21                                                                              -10                                                                              -16                                                                              -28                                                                              -20                                                                              -31                                        18    Avg. 2.5                                                                              9.5                                                                              8.0                                                                              10.5                                                                             11.8                                                                             11.0                                                                             12.0                                                                             15.0                                                Max. 3.0                                                                              18.0                                                                             12.0                                                                             12.0                                                                             13.0                                                                             13.0                                                                             14.0                                                                             18.0                                                % Red.                                                                             4  -78                                                                              13 12 14 26 19 5  2                                          22    Avg. 0.0                                                                              0.0                                                                              0.0                                                                              0.5                                                                              1.5                                                                              0.3                                                                              0.0                                                                              0.0                                                 Max. 0.0                                                                              0.0                                                                              0.0                                                                              2.0                                                                              5.0                                                                              2.0                                                                              0.0                                                                              0.0                                                 % Red.                                                                             100                                                                              100                                                                              100                                                                              96 89 98 100                                                                              100                                                                              98                                         23    Avg. 2.8                                                                              5.3                                                                              7.6                                                                              7.3                                                                              7.8                                                                              3.3                                                                              9.5                                                                              13.3                                                Max. 5.0                                                                              9.0                                                                              9.0                                                                              11.0                                                                             10.0                                                                             12.0                                                                             15.0                                                                             16.0                                                % Red.                                                                             -6 0  16 39 43 78 37 16 28                                         __________________________________________________________________________

EXAMPLE V

A fifth trial was performed wherein five hollow step wedges were cast.The systems tested, along with materials and process configurations, arelisted in Table VII. The systems tested included three yttria-basedcorecoats of the present invention (corecoat nos. 2, 13, 20). The trialwas conducted in the same manner as Example IV. Results for eachcore/corecoat system are given in Tables VII and VIIA. Again a prior artzirconia-based corecoat was used as a baseline. The yttria used in thecorecoat nos. 2 and 13 was fused grain yttria powder having a density of5.00 gm/cc. The yttria used in the corecoat no. 20 was a nonfused,highly calcined, large grain size yttria powder having a density of 5.00gm/cc. The Ti-ester binder used in corecoat nos. 13 and 22 wasspecifically Titanate Binder LPC 3851/1, atitanium-acetylacetonate-butoxide derivative manufactured by DynamitNobel (distributed by Dynamit Nobel of America, Inc., Kay-Fries, Inc.,Chemical Division). The core coating formulation used in corecoat no. 13was as follows:

Yttria Powder (Fused Grain, -325 mesh): 260 gm,

Titanate Binder LPC 3851/1: 60 ml,

Dow Chemical DOWANOL®PM (propylene glycol methyl ether): 15 ml.

                                      TABLE VII                                   __________________________________________________________________________    CORE COATING EVALUATION - EXAMPLE V                                                                           Avg. Alpha-Case                                                               Reduction Vs.                                 ID                                                                              Core Coating         Coating Fire                                                                           Baseline (%)*                                 __________________________________________________________________________     1                                                                              Fused ZrO.sub.2 /30% SiO.sub.2 Sol                                                                 1850° F./1 hr/air                                                               Baseline                                       2                                                                              Fused Y.sub.2 O.sub.3 /Ethylsilicate (13% SiO.sub.2)                                               2050° F./1 hr/air                                                               97                                            13                                                                              Fused Y.sub.2 O.sub.3 /Ti-ester (26% TiO.sub.2)                                                    2250° F./2 hr/air                                                               97                                            20                                                                              Nonfused, Dense Y.sub.2 O.sub.3 /Et. Sil. (13% SiO.sub.2)                                          2050° F./1 hr/air                                                               97                                            22                                                                              Fused ZrO.sub.2 /Ti-ester (26% TiO.sub.2)                                                          2250° F./2 hr/air                                                                6                                            __________________________________________________________________________     Ti-ester Binder = Titanate Binder LPC 3851/1                                  All coatings applied by aerosol spray                                         All molds were fired 1850° F./1 hr/air                                 *Average for 7 section thicknesses                                       

                  TABLE VII                                                       ______________________________________                                        CORE COATING EVALUATION - EXAMPLE V                                           ALPHA-CASE DEPTHS (MILS)                                                      AND AVG. ALPHA-CASE REDUCTIONS (%)                                                   Section Thickness                                                      ID           1/8"   1/4" 3/8" 1/2" 5/8" 3/4" 7/8" Avg.                        ______________________________________                                         1    Avg    6.3    10.9 13.1 15.6 17.1 19.1 21.3                             (Base-                                                                              Max    8.0    12.9 15.3 18.0 19.3 24.7 25.3                             line                                                                           2    Avg     0      0   0.1  0.4  0.5  1.2  1.6                                    Max     0      0   0.3  1.3  0.8  1.7  2.4                                    %      100    100  99   97   97   94   92   97                                Red.                                                                    13    Avg     0      0    0   0.1  0.5  1.1  2.0                                    Max     0      0    0   0.3  1.3  1.9  2.8                                    %      100    100  100  99   97   94   91   97                                Red.                                                                    20    Avg     0      0   0.1  0.8  0.6  0.8  1.4                                    Max     0      0   0.5  3.0  1.8  1.1  1.9                                    %      100    100  99   95   96   96   93   97                                Red.                                                                    22    Avg    6.0    10.2 12.4 14.2 16.2 17.5 20.0                                   Max    8.0    13.7 15.7 16.7 20.7 20.0 22.7                                   %       5      6    5    9    5    8    6    6                                Red.                                                                    ______________________________________                                    

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided that they come within the scope ofthe appended claims and their equivalents.

What is claimed is:
 1. A method of fabricating molds for casting reactive metals comprising the steps of: preparing an yttria-based slurry comprising a dense grain yttria powder having a density greater than 4.60 gm/cc and a non-aqueous-based binder; and using said slurry as a mold facecoat or corecoat in the fabrication of a mold for casting a reactive metal.
 2. The method of claim 1 wherein said yttria powder has a density greater than 4.90 gm/cc.
 3. The method of claim 2 wherein said yttria powder is a fused grain yttria powder having a density of about 5.00 gm/cc.
 4. The method of claim 1 wherein said non-aqueous-based binder includes an organometallic compound containing one or more metals selected from the group of silicon, titanium, zirconium, aluminum and the rare earth elements.
 5. The method of claim 4 wherein said organometallic compound is a metal alkoxide, a chelate or contains mixed alkoxide-chelate ligands.
 6. The method of claim 5 wherein said organometallic compound is ethyl silicate.
 7. The method of claim 5 wherein said organometallic compound is a titanium-acetylacetonate-butoxide derivative.
 8. The method of claim 1 wherein said non-aqueous-based-binder includes a drying control additive.
 9. The method of claim 1 wherein said yttria powder is a fused grain yttria powder and said non-aqueous-based binder comprises ethyl silicate and propylene glycol methyl ether.
 10. The method of claim 1 wherein said mold is for casting a gas turbine engine component.
 11. The method of claim 1 wherein said mold is for casting a surgical implant.
 12. The method of claim 1 wherein said mold is for casting a chemical resistant component.
 13. A method of making a casting shell for fabricating a reactive metal casting comprising the steps of:preparing a pattern; dipping said pattern in an yttria-based slurry comprising a dense grain yttria powder having a density greater than 4.60 gm/cc and a non-aqueous-based binder; forming a shell on said dipped pattern; drying said shell; removing said pattern; and firing said shell.
 14. The reactive metal casting made from the casting shell made by the method of claim
 13. 15. The reactive metal casting of claim 14 wherein said reactive metal is a titanium alloy.
 16. The reactive metal casting of claim 14 wherein said casting is a gas turbine engine component.
 17. The reactive metal casting of claim 14 wherein said casting is a surgical implant.
 18. The reactive metal casting of claim 14 wherein said casting is a chemical resistant component.
 19. A method of making a casting core for fabricating a reactive metal casting comprises the steps of:forming a removable ceramic core; coating said core with an yttria-based slurry comprising a dense grain yttria powder having a density greater than 4.60 gm/cc and a non-aqueous-based binder; and firing said coated core.
 20. The reactive metal casting made with the casting core made by the method of claim
 19. 21. The reactive metal casting of claim 20 wherein said reactive metal is a titanium alloy.
 22. The reactive metal casting of claim 20 wherein said casting is a gas turbine engine component.
 23. The reactive metal casting of claim 20 wherein said casting is a surgical implant.
 24. The reactive metal casting of claim 20 wherein said casting is a chemical resistant component. 